The present invention relates to resistance measuring apparatus and particularly to simplified and foolproof instrumentation for implementing a four-terminal ohmmeter.
In a simple circuit for resistance measurement, the ratio of the voltage across a pair of instrument terminals to the current flowing therethrough is determined. Since the actual device under test may be at some distance from the measuring instrument, the instrument inherently measures a resistance including a variable value accounting for the intervening test leads. A more accurate determination can be provided by a four-terminal ohmmeter wherein the current and voltage factors are separated.
Referring to FIG. 1, schematically illustrating a four-terminal ohmmeter, a source (i.e., current source) 10 is connected by test leads represented by resistances 12 and 14 to opposite terminals of an unknown resistance 16. A sensing means, here comprising voltmeter 18, is coupled separately via test leads here represented by resistances 20 and 22 to the same terminals of the unknown resistance 16. It is understood that voltmeter 18 is customarily a very high impedance device and therefore the current through and the voltage drop across resistances 20 and 22 are very small. Therefore, the only substantial current flowing through unknown resistance 16 is the predetermined current from source 10, while the voltage across unknown resistance 16 is accurately measured by voltmeter 18, wherein the value of the unknown resistance is equal to the aforementioned voltage divided by the aforementioned current.
In use, the four-terminal ohmmeter requires two pairs of test leads, i.e., the pair 12, 14 from the source, and the separate pair 20, 22 from the sensing meter. Test probes, clips, or the like at the ends of these test leads are applied as closely as possible to the device the resistance of which is to be measured.
While the foregoing four-terminal method is accurate, it is awkward in its implementation and can be intimidating to personnel not familiar with the device.
It is accordingly an object of the present invention to provide an improved resistance measuring apparatus for making highly accurate resistance measurements in an automatic and foolproof manner.
It is another object of the present invention to implement a four-terminal ohmmeter in a manner that is easily employed and backwards compatible with conventional test lead sets.